Recently, ones utilizing a digital system are noted in the technology of image formation by electrophotography. For example, a dot image formed by small dots on 1200 dpi level (dpi is the number of dot per inch) can be finely reproduced by employing a toner having a small diameter of several micrometers order.
It has been made possible to form a beautiful full color toner image by the use of color toners of yellow, magenta, cyan and black, which is no way inferior to a silver salt photographic image. There is a great need for reproducing a high precision digital image with high fidelity. A technique is applied as one of the countermeasures corresponding to the requirement, in which such the need is satisfied by controlling the physical properties of the toner to satisfy.
For example, there is a technique in which a composite oxide compound of strontium titanate having an average primary particle diameter of from 300 nm to 5400 nm is added as an external additive to a black toner to be used for forming an image. (For example, refer patent publication No. 3).
The purpose of the prior technique is to prevent filming and to raise the visible glossiness by addition of an external additive having large diameter to a polymerized toner. In concrete, a smaller amount of the large diameter external additive is added to the black toner and a larger amount of the additive is added to the other color toners.
It is found, however, that the following problems are caused by the addition of the large diameter external additive.
(a) Lowering in the reproducibility of the second color
(b) Lowering in the fixing ability on the folded line
(c) Occurrence of filming and fault in cleaning on the occasion of printing of a black document image (characters image)
Solution of the above problems caused by the use of the large diameter external additive is demanded in the present field of the technology.
In the formation of the full color image, the consumption of the toner is larger compared with the formation of a monochromatic image because of high printing area ratio. Consequently, the probability of that the toner is employed to development just after supplied to the developing device is raised. Besides, a compact size printer is popular with customers, and need for a compact size printer not occupying excessive space is high. Therefore, it is inevitable to miniaturize the size of the developing device.
Consequently, a property is required to the toners for full color image formation that the toners can rapidly obtain the sufficient charging ability for employing the image formation after the supplement into the developing device.
As a method for satisfying the above requirement, a technique has been disclosed that a slipping agent such as zinc stearate is added for accelerating the rising up of the charging ability of the toner so as to prevent the fogging of image at the initial period of the use of the toner and the dirt on the apparatus caused by the scatter of the toner (for example, cf. patent publication 1).
Moreover, a technique has been disclosed that the slipping agent is added in a same amount for each of spherical toner particles of yellow, magenta, cyan and black to provide the same slipping ability to each or the toner for improving the cleaning property of the toners (for example, cf. patent publication 2).
It is found, however, that the color image tends to be made white turbidity without clarity when the image is formed by the use of the color toner containing the slipping agent since the fusion between the toner particles is hindered on the occasion of the fixation of the toner transferred onto a recording medium.
Particularly, the degradation in the color reproducibility of the secondary color formed by the plural kinds of the toner tends to be considerable.
It is supposed that such the problem is caused by that the fusing ability of the toner at the fixation is hindered because the slipping agent added to the toner has higher melting point than that of the toner particle.    [Patent Publication 1] Japanese Patent Publication Open to Public Inspection, hereinafter referred to as Japanese Patent O.P.I. Publication, No. 2001-92173 (cf. Paragraph 0009)    [Patent Publication 2] Japanese Patent O.P.I. publication No. 2002-311775 (cf. Paragraphs 0084-0085)    [Patent Publication 3] Japanese Patent O.P.I. publication No. 2002-372808